The present invention relates generally to appliances used in dentistry and is more particularly concerned with a new and improved appliance system used principally in orthodontics but also having application to prosthetics and oral surgery.
As is well known, orthodontic appliances are used to move or manipulate certain teeth to correct irregularities and/or abnormalities in their relationships with surrounding members. This is achieved by the application of systems of force that have their origin primarily in elastically-deformed wires which absorb and release energy during loading and unloading. Heretofore, the force-imparting wires used in orthodontic treatment have been made from certain metal alloys, such as 18-8 stainless steel, chrome-cobalt-nickel (CCN) alloys, such as those sold under the tradename "Elgiloy" and, more recently, from titanium containing alloys that take advantage of the bending and torsional properties of those materials. With the earlier materials efforts were directed almost exclusively toward the development of optimum appliance configurations with only ancillary consideration being given to the material used for the appliances.
Proper application of the correct forces requires not only the study of suitably contoured and clinically dimensioned shapes or configurations together with variations in the cross-sectional dimensions of the force-imparting wire, but also a better understanding of the biomechanism involved in orthodontic appliances.
Desirable tooth movement can best be achieved by producing an optimal force system capable of delivering relatively light but continuous corrective forces. The primary or basic biomechanical characteristics include a moderate to low force magnitude whereby the teeth will move rapidly and relatively painlessly with minimum tissue damage, a constant force level over time as the appliance experiences deactivation in order to provide maximum tissue response, an accurate location of the point of application of the force or its equivalent and a uniformity in the force applied through the total distance over which the force acts. It also is desirable to provide within an orthodontic appliance the ability to undergo large deflections without deformation. Of course, if the force acting on the teeth decays too rapidly, the teeth will move more slowly and it becomes more difficult to accurately produce the desired effect.
Heretofore, the force magnitude applied to the teeth is produced in part by the cross section of the wire used in the appliance, with smaller wires providing the desired lower or reduced force. Primarily, however, the smaller wires are used to achieve large deflections. As will be appreciated, larger wires fit well in the slots of band-mounted or direct bonded brackets and a good fit is necessary for controlled tooth movement. If the wire and the bracket are not properly matched, the play between those members leads to loss of control. Reduction in slot or lumen size may be undesirable since (1) it may be more difficult to control tolerances and (2) manufacturing variations in alloy wire cross section have a proportionately greater effect on force magnitudes. Despite this, a reduction in the wire cross section with its attendant reduction in load-deflection rate and increased range historically has been the course followed to achieve force constancy using 18-8 stainless steel wire. In this connection, care must be taken since too great a reduction in cross section can result in permanent deformation before optimal forces are reached.
Although the principal and predominant emphasis in orthodontic research has been on improved appliance design, and relatively little attention has been given to alternatives for the conventionally employed 18-8 stainless steel or the chrome-cobalt-nickel alloy wires, efforts are now being made in providing the aforementioned desirable biomechanical characteristics through the use of alternative titanium alloy materials. One example of such an approach can be found in the proposed utilization of Nitinol alloys of the type described in U.S. Pat. No. 3,351,463. These materials are near-stoichiometric intermetallic compounds of nickel and titanium, preferably having cobalt substituted for the nickel on an atom to atom basis. The alloy can be preformed below its critical transition temperature and, when heated to above that temperature, will display a mechanical memory causing the material to return to its predisposed shape. The application of this material to orthodontics is set forth in U.S. Pat. No. 4,037,324 where the longitudinal shrinkage characteristic of the wire is used. Although this intermetallic material is reported to be quite ductile, it has been found in practice that the material will not withstand cold bending into major orthodontic configurations and cannot be used for closing loops and the like. This, of course, severely limits the alloy's use in the formation of appliances that require significant bends in their design. Additionally, the material cannot be welded or soldered, thereby substantially hampering its utilization. In fact, the only way to join this wire, Nitinol, is by mechanical means. Furthermore, metal wires such as these are not aesthetically pleasing and it is difficult to product specialized shapes and cross sections from this material.
Our prior development using beta titanium alloys as reported in U.S. Pat. No. 4,197,643 solved many of the problems encountered when using stainless steel or Nitinol while, at the same time, facilitating the delivery of optimum orthodontic forces. That material can be easily joined, exhibits a low stiffness or modulus of elasticity while providing a preferred low force magnitude and force constancy over a prolonged period of time to achieve continuous, relatively painless tooth movement with maximum tissue response and minimum tissue damage. However, the material lacks optimum formability for providing customized shapes for a particular situation and is not aesthetically pleasing.
Although orthodontists are now beginning to control force systems through the use of material selection as well as by appliance design, the properties of the available materials limits their use. In this connection, there are several aspects or clinical parameters which are important for all orthodontic appliances and relate to both the wire and the attachments which include the brackets and tubes. For the wires these include springback, stiffness, formability, joinability and aesthetics as well as shape and dimension. The characteristics of the attachments involve considerations such as geometry, bonding to the teeth, attachment of wires, aesthetics and ease of manufacture.
The elastic deformation or springback parameter is a measure of the amount of deflection or activation which the appliance can sustain and still be totally elastic, that is, recover to its original shape and position. This feature is important because it determines the distance over which an appliance can effectively act before readjustment by the orthodontist is necessary. Appliances which can sustain larger deflection can more readily engage teeth which are more severly malopposed. The elastic deflection or springback of an appliance is fundamentally proportional to its ratio of flexure strength to flexure modulus or similarly its ratio of tensile yield strength to modulus of elasticity.
The stiffness of an appliance is important because it is a primary determinant of the force which will be applied to the teeth. Greater stiffness results in more force for each unit of activation. Appliance stiffness is proportional to the material's flexure modulus and for isotropic materials, its tensile modulus. Thus materials of lower modulus of elasticity have a lower stiffness.
In most clinical situations, wire or appliances have to exhibit sufficient formability to be formed to a desired customized shape for a particular case. Additionally, they have to be joined while retaining their strength and elasticity characteristics, and they must be of the desired cross-sectional shape and dimension.
Of course, aesthetics also is an important consideration, particularly for all labial appliances. All metal wires and brackets are gray or silver in color and are quite obvious against the white background of the tooth structure. Consequently, the use of clear or tooth-colored appliances would be considerably more aesthetically pleasing to many patients.
The brackets, tubes, ligature wires and related attachment components that translate the force from the wire directly to the tooth also have various characteristics which have to be considered for any orthodontic appliance. For example, the design, geometry and overall dimensions of the attachment are important for both its ease of manipulation as well as its ability to help contribute to the active aspects of the orthodontic force system. Attachments are often bonded directly to the tooth surface without the use of a band which circumscribes the entire tooth. An attachment which is bonded directly requires certain functional shapes and contours on the surface which will contact the tooth in order to obtain satisfactory bonding. Further the attachments should be easy to fabricate or manufacture.
Unfortunately, the alloys that have a high elastic deflection, such as the nickel titanium alloys, cannot be readily formed and can be joined only by mechanical means. On the other hand, stainless steel which probably has the lowest elastic deflection, is very formable and can be joined by soldering, albeit this is an operator sensitive procedure.
It is also important to note that nuclear magnetic resonance diagnostic testing is becoming more popular and there are some predictions that in the future it may replace the CAT scan in certain diagnostic procedures. Metallic orthodontic appliances of the type that have been used heretofore have specifically been identified as a problem area for this diagnostic procedure since the metal does not exhibit the requisite radiolucency and interferes with the resulting images.
Attempts have been made to fabricate some brackets from polycarbonate and ceramic materials in an attempt to obtain a more aesthetic appliance. However, the polycarbonate brackets cannot resist the high stress magnitudes frequently encountered in orthodontics and typically fail at torque levels well below the levels obtainable with conventional stainless steel arch wires. The ceramics are expensive, not available in the more complex shapes and sizes and are brittle.
Accordingly, it is an object of the present invention to provide a new and improved orthodontic appliance system that facilitates the application of a given force with greater ease and accuracy while customizing both the stiffness and strength characteristics of the appliance thereby providing an associated increase in the effective working time of the appliance while meeting the necessary criteria of biocompatibility, formability, environmental stability, aesthetics and ease of joining and bonding.
Another object of the present invention is to provide a dental appliance system of the type described that provides improved springback coupled with the ability to change or vary the stiffness of the wires without changing their size and allow for the fabrication of unique wire/bracket engagement designs using facile manufacturing techniques.
Still another object of the present invention is to provide an orthodontic appliance system of the type described that utilizes a new and improved force-imparting material capable of exhibiting a lower modulus of elasticity than prior alloys, high elastic deflection and a selective ratio of yield strength to modulus of elasticity while reducing the need for periodic installation of wires of varying cross section. Included in this object is the provision for the use of a material permitting more compatible cross section, shape and dimension in both the wires and the attachments while minimizing the need for closer wire tolerances. Also included is the provision for wires whose force magnitudes and moment to force ratios are controlled by selection of the modulus of elasticity rather than the traditional approach of simply modifying the cross section thereby providing an expanded sequence of wires that would make "constant cross section orthodontics" clinically feasible.
Still another object of the present invention is to provide a new and improved dental appliance system of the type described that utilizes materials covering a broader spectrum of desired characteristics and is capable of being formed into a wide array of orthodontic appliances from the simple to the highly complex orthodontic configurations in order to deliver the optimum moment to force ratios, improved control through early and accurate bracket engagement, ease of handling, accurate centers of rotation of the tooth as it is moved and simplified attachments and instrumentation. Included in this object is the provision for the use of a composite material capable of taking advantage of the qualities of its separate components coupled with signficantly enhanced aesthetic characteristics.
A further object of the present invention is to provide a new and improved dental appliance system of the type described which utlizes fiber reinforced composite materials of excellent formability, joinability and appearance while simplifying manufacturing operations. These materials can be used not only for simple wires but also for many components of intraoral fixed appliances which include arches, segments, hooks, tiebacks, ligature wires and springs, pins, brackets, tubes, active lingual appliances and other mechanisms. They may also be used for removable and extraoral appliances such as headgears.
A still further object is to provide a system of the type described that employs fiber reinforced composite material and includes the ability to deliver lower forces for a given deflection and more constant force levels with time due to a lower load-deflection rate, to change wire stiffness over a continuous range without changing the cross-sectional dimensions of the wire, to allow engagement of more severe dental malrelations and increase the "working time" or "working range" of the appliance and to increase the ease and accuracy of applying a given force. Included in this object is the provision for system designs that can be coordinated with the material properties to optimize each and facilitate the use of unique and more convenient shapes.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects are achieved in accordance with the present invention by providing an orthodontic appliance fabricated from a fiber reinforced composite material with the force delivery component having a flexure modulus of elasticity between 0.3.times.10.sup.6 and 30.times.10.sup.6 psi and a ratio of yield strength to modulus of elasticity up to at least about 40.times.10.sup.-3. The composite consists essentially of a polymeric matrix and a fiber component embedded within the matrix with the fiber component constituting greater than 5 percent by weight of the composite material. The system is particularly efficacious in providing materials having moduli of elasticity beyond the scope of those provided by the metal alloys utilized heretofore coupled with desirable strength characteristics and improved aesthetic qualities.
A better understanding of the invention will be obtained from the following detailed description and the accompanying drawing as well as from the illustrative applications of the invention including the several components of the invention and the relation of one or more of such components with respect to each of the others as well as to the features, characteristics, compositions, properties and relation of elements described and exemplified herein.